1. Technical Field
The present disclosure relates, generally, to heat transfer systems, including heating, ventilation, and air condition (HVAC) systems and controls for such systems. More specifically, this disclosure provides apparatus, including electronic thermostats, for detecting temperatures and elapsed times and, in response to those temperatures and elapsed times, controlling the opening or closing of an electric circuit that provides electricity to a component connected to that electric circuit.
2. Description of the Related Art
Air conditioning, refrigeration, and other heat transfer systems include four core components: a compressor, a condenser, an expansion device, and an evaporator, each of which is in operable connection through a fluid circuit of refrigerant such as, for example, a chlorofluorocarbon (CFC), a hydrochlorofluorocarbon (HCFC), and a hydrofluorocarbon (HFC).
In a vehicle air conditioning system, such as an automobile or truck air conditioning system, the refrigerant is typically in heat exchange with ambient air within the vehicle compartment by means of the evaporator, or endothermic heat exchanger. As the liquid refrigerant passes through the evaporator, it absorbs heat from the ambient air and converts into a gaseous state. The gaseous refrigerant leaving the evaporator is drawn through a suction line into the compressor, which increases the gas pressure before it passes through the condenser, or exothermic heat exchanger, where it gives off heat and cools back to a liquid state under high pressure. The high pressure liquid refrigerant then passes through an expansion device, such as an expansion valve, wherein the fluid pressure is adiabatically decreased prior to re-entering the evaporator to repeat the cycle.
Compressors are typically connected to a control device that controls whether the compressor is operational. The compressor control device can be a clutch, which comprises an electromagnet that, when engaged, produces a strong magnetic field when current is applied thereby activating the compressor. The compressor control device can also be a fluid flow valve, which controls the flow of a fluid to a hydraulic compressor.
When the control device is disengaged, the compressor stops functioning; when the control device is engaged, the compressor starts functioning. When the compressor is operational, it takes in and compresses cool, low pressure gas from the evaporator and discharges hot, high pressure gas, which passes to the condenser where heat is released by condensation, which thereby converts the high pressure gas to a high pressure liquid that passes through the expansion valve (generally a narrowing inlet) that converts the high pressure liquid refrigerant to a gaseous state before returning to the evaporator.
In contrast, when the compressor in not operational, it cannot compress the cool, low pressure gas exiting the evaporator into hot, high pressure gas, which limits the release of heat by condensation. The low pressure liquid that passes through the expansion valve, therefore, remains a low pressure liquid as it enters through the evaporator, which is ineffective in extracting heat from the ambient air.
Heat absorption by the refrigerant at the evaporator causes the evaporator to cool. As the temperature of the evaporator falls, water vapor from the atmosphere surrounding the evaporator condenses on the evaporator. If the evaporator temperature falls below the freezing point of water, the condensation on the evaporator freezes and begins to accumulate. If the evaporator remains below the freezing point of water for an extended period of time, the ice accumulation on the evaporator can become excessive thereby insulating the evaporator and reducing the efficiency of heat transfer.
Heat exchange systems often employ one or more operating control device(s) to regulate on and off cycling and to maintain desired system temperatures and pressures. Heat exchange systems also frequently employ one or more safety control(s) to stop system operation under unsafe or undesirable conditions. Depending upon the precise application, heat exchange system operating control device(s) respond to temperature, pressure, humidity, liquid levels, input from other controls, and manual intervention.
Heat exchange systems can employ an operating control device that detects the temperature at one or more system component(s). For example, a heat exchange system can employ an operating control device that detects the temperature of an evaporator and, in response to a predetermined temperature at the evaporator core, controls the operation of an electric circuit that supplies electricity to another system component, such as a compressor control device.
In a typical heat exchange system, such an operating control device detects a predetermined lower evaporator temperature (referred to as a cut out point) and, in response to that predetermined cut out point, which is typically at or below the freezing point of water, opens the electrical circuit that provides electricity to the compressor control device, thereby causing the control device to disengage and, consequently, turning off the system compressor. As a result, the evaporator temperature rises until it reaches a predetermined upper temperature (a cut in point) and in response to the cut in point, the control device closes the electrical circuit that provides electricity to the compressor control device causing it to engage and, consequently, turning on the system compressor. The compressor stays on until the evaporator temperature reaches the cut out point and the cycle then repeats.
A thermostat is a conventional operating control device employed in heat transfer systems to detect and respond to changes in temperature, such as predetermined lower and upper evaporator temperatures at an evaporator core. A thermostat is a thermally activated switch that employs a temperature dependent element, e.g., a probe, which responds to changes in temperature and communicates those temperature changes to an electrical switch mechanism, which controls the flow of electricity to a downstream device, such as compressor control device.
Conventionally, thermostats for use in heat exchange systems are mechanical devices that utilize a probe that includes a capillary tube containing a refrigerant that expands or contracts in response to changes in temperature. The probe is in operable connection with a thermostatic switch mechanism that includes contact points that open at a specified lower temperature due to contraction of the capillary tube refrigerant and close at a specified upper temperature due to refrigerant expansion. When the thermostat contact points open, the electrical circuit breaks and, when the thermostat contact points close, the electrical circuit is restored.
U.S. Pat. Nos. 5,083,437 and 5,162,774 describe a remotely changeable thermostat for a refrigeration or air conditioning system that has a fluid filled capsule with a diaphragm that is movably responsive to fluid expansion and contraction within the capsule and actuates a compressor power switch via a lever means. A separate remote user activated switch energizes an electrical actuator for moving the fulcrum of a lever means to change the sensed temperature of the evaporator at which the power switch is actuated.
More recently, mechanical thermostats are being replaced with electronic thermostats that, like the mechanical thermostats, are thermally actuated switches that open and close an electrical circuit that controls a downstream device. In an electronic thermostat, the functionality of the capillary tube and contact points is achieved by a thermistor, such as a positive temperature coefficient (PTC) or a negative temperature coefficient (NTC) resistor, and a control circuit that amplifies thermistor resistance and electronically activates and deactivates as a function of temperature at the thermistor, thereby opening and closing the electrical circuit that controls a downstream device.
Thermostats that are used in conjunction with heat exchange systems, including portable refrigerators and air conditioning systems, have a limited life-span and, therefore, often must be replaced during the useful life of the heat exchange system with which they are used. Consequently, such thermostats are frequently configured for ease of accessibility and replacement by a service technician or end-user.
It is well understood that frequently-repeated on and off cycles, i.e., short cycling, exert wear and tear on the compressor and reduces its operational life. It is, therefore, desirable to be able to control the time elapsed from when a compressor is turned on and when it is turned off without regard to whether the evaporator first reaches a cut in point or a cut out point, respectively. This functionality can be achieved by employing a thermostat that not only detects evaporator temperature but also tracks the elapsed time between cut in and cut out points and responds to the cut in and cut out points secondarily to responding to the elapsed time between cut in and cut out points.